The mechanism by which arginine deiminase from Mycoplasma arthritidis catalyzes the hydrolysis of the guanidino group presents a unique challenge to the enzyme chemist. We plan to establish the major aspects of the enzymic mechanism. First, the identification of the essential residue involved in nucleophilic catalysis is being studied by labeling with active-site-directed inhibitors. Second, the details of the individual steps in the mechanism are under investigation. Third, the functional significance of the subunit structure to catalytic activity and control by modifiers is being sought in detailed kinetic studies.